Tunable optoelectronic frequency filter

ABSTRACT

A wavelength tunable optolelectronic device. Two channel waveguides are provided in an optoelectronic structure made of a ferroelectric material. The wave-guides are adjacent along at least a coupling region. A periodically poled structure is provided in the coupling region, and at least one of the waveguides is untouched by this structure. An electric field is applied in the coupling region, through both waveguides, to enable coupling a light of a given wavelength between the two waveguides. The amplitude of the electric field may be selected to tune the coupling wavelength. A third waveguide or more having a coupling region in common with the second waveguide may also be added to allow the bandwidth of the resulting beam to also be tuned.

FIELD OF THE INVENTION

[0001] The present invention relates to optoelectronic devices and moreparticularly concerns such a device, simple in construction, and wherethe coupling wavelength between two waveguides is tunable.

BACKGROUND OF THE INVENTION

[0002] Optical devices such as wavelength add/drop filters, bandbassfilters, directional couplers, etc. are crucial elements of opticalcommunication systems. They are mainly used in DWDM (Dense WavelengthDivision Multiplexing) applications, where efficient adding and droppingof channels is essential. In this context, a wavelength tunableadd/drop/filter is very advantageous since it allows networkreconfiguration. Such a device is also useful for wavelength routing ofthe signal. This characteristic is even more important for metro oraccess DWDM optical networks where reconfigurations are constant. Themarket of wavelength tunable bandpass filters is also important, wherethere is a great advantage to use a tunable filter with fast responsetime, integrated and with no moving parts (electronic control). An evenmore advantageous feature of a such a wavelength tunable device is thatit may serve as the main building block of an integrated OADM (OpticalAdd/Drop Multiplexer) if it is combined with, or integrated to, theproper wavelength converter.

[0003] A wavelength tunable device is mentioned in U.S. Pat. No.5,887,089 (Deacon et al). Deacon teaches a structure made of aferroelectric material having good optoelectronic properties providedwith channel waveguides therein. In one embodiment, shown in FIG. 10 ofthe above mentioned patent, where two adjacent waveguides lie in thestructure and are provided with a periodically poled structure extendingover both of them. Electrodes are provided on either side of thecoupling region. When an electric field is applied between theelectrodes, the refractive index grating defined by the poled structureis turned on, and coupling is allowed between the two waveguides forlight of a given wavelength, determined by the propagation constants ofthe waveguides and the period of the grating.

[0004] In the above-mentioned patent, Deacon explores at length thepossibility of tuning the coupling wavelength of such a device. Toachieve such a result, one must operate an average refractive indexchange in the coupling region. To this end, Deacon suggests severaltechniques, such as using, in the periodic structure, alternate domainsof optoelectronic and non-optoelectronic material, using an asymmetricgrating to obtain a duty cycle different than 50%, depositing anadditional optoelectronic layer over the basic structure, etc. All ofthe proposed solutions however involve a more complex and costlymanufacturing process for the resulting device.

SUMMARY OF THE INVENTION

[0005] It is therefore an object of the present invention to provide asimple wavelength tunable optoelectronic device.

[0006] It is a secondary object of the present invention to provide sucha device also allowing a tuning of the signal bandwidth.

[0007] Accordingly, the present invention provides a wavelength tunableoptoelectronic device, including an optoelectronic structure made of aferroelectric material. First and second channel waveguides are providedin the optoelectronic structure. The two waveguides are adjacent atleast along a coupling region. A periodically poled structure isprovided in this coupling region, and at least one of the first andsecond channel waveguides is untouched by this periodically poledstructure.

[0008] The device also includes means for generating an electric fieldof selectable amplitude in the coupling region, through both the firstand second waveguides. The electric field enables a coupling of light ofa coupling wavelength between the first and second waveguides in saidcoupling region. The amplitude of the electric field in the waveguideuntouched by the periodically poled structure determines the couplingwavelength.

[0009] According to an alternative embodiment of the present invention,there is also provided a wavelength and a bandwidth tunableoptoelectronic device.

[0010] The device includes an optoelectronic structure made of aferroelectric material. First, second and third channel waveguides areprovided in this structure.

[0011] The first and second waveguides are adjacent at least along afirst coupling region. A first periodically poled structure is providedin the first coupling region. At least one of the first and secondchannel waveguides is however untouched by the first periodically poledstructure.

[0012] Similarly, the second and third waveguides are adjacent at leastalong a second coupling region, a second periodically poled structurebeing provided in the second coupling region, at least one of the secondand third channel waveguides being untouched by said periodically poledstructure.

[0013] Means are provided for generating a first electric field ofselectable amplitude in the first coupling region, through both thefirst and second waveguides. The first electric field enables a couplingof light of a first coupling wavelength and first bandwidth between thefirst and second waveguides, in said first coupling region. Theamplitude of the first electric field in the waveguide untouched by thefirst periodically poled structure determines the first couplingwavelength.

[0014] Means for generating a second electric field of selectableamplitude, in the second coupling region, through both the second andthird waveguides, are also provided. The second electric field enables acoupling of light of a second coupling wavelength and a second bandwidthbetween the second and third waveguides in the second coupling region.The amplitude of the second electric field in the waveguide untouched bythe second periodically poled structure determines the second couplingwavelength.

[0015] In this manner, the device enables a coupling of light of atunable wavelength and tunable bandwidth from the first to the thirdwaveguides.

[0016] Other features and advantages of the present invention will bebetter understood upon reading the description of preferred embodimentsthereof with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a schematic drawing of an optoelectronic deviceaccording to a first preferred embodiment of the invention.

[0018]FIG. 2 is a schematic drawing of an optoelectronic deviceaccording to a second preferred embodiment of the invention.

[0019]FIG. 3A is a diagram showing the wavelength distribution at thecoupling between the first and second waveguides of FIG. 2; FIG. 3B is adiagram showing the wavelength distribution at the coupling between thesecond and third waveguides of FIG. 2; and FIG. 3C is a diagram showingthe resulting wavelength and bandwidth of light coupled from the firstto the third waveguides of the device of FIG. 2.

[0020]FIG. 4 shows the spectral distribution for a device according tothe embodiment of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0021] Referring to FIG. 1, there is shown a wavelength tunableoptoelectronic device 10 according to a first preferred embodiment ofthe invention.

[0022] The device 10 is built into an optoelectronic structure 12, whichis made of a ferroelectric material. In the preferred embodiment, thismaterial is a LiNbO₃ crystal 14 having good optoelectronic properties inits x-cut propagated along y direction. A first and a second channelwaveguide 18 and 20 are provided in the optoelectronic structure 12. Thechannel waveguides are preferably singlemode waveguides, and may be madefor example by titanium in-diffusion or proton exchange. The twowaveguides 18 and 20 preferably have different widths so that theirpropagation constants are different at rest. They are adjacent along atleast a portion thereof, defining a coupling region 24.

[0023] A periodically poled structure 22 is provided in the couplingregion 24. It may extend in any appropriate portion of the couplingregion where it will allow light coupling between the two waveguides 18and 20, but at least one of these waveguides should be untouched by theperiodically poled structure 22, for reasons explained below. Known highvoltage periodic poling techniques may be used to make the poledstructure 22, such as those disclosed in U.S. Pat. No. 5,193,023(Masahiro et al) and in co-pending application Ser. No. 09/796,832 filedon Mar. 1, 2001. In the illustrated embodiment, the periodically poledstructure 22 extends over almost all of the first waveguide 18, which isparallel and proximate to the second waveguide 20 along its entirelength. However, for other embodiments it may be advantageous to havethe periodically poled structure 22 on only a portion of one of thewaveguides. It may also extend in between the two waveguides, partly onone waveguide and in between the waveguide, etc, but one of thewaveguides should always be free of contact with the periodically poledstructure 22.

[0024] The device 10 includes means for generating an electric field inthe coupling region 24, preferably embodied by a pair of electrodes 34.In the illustrated embodiment, the two electrodes respectively extendover and under the optoelectronic structures on either side of thecoupling region 24. The electric field extends through both waveguides18 and 20. Its effect is two-fold. First, it will photo-induce a phasematching grating in the periodically poled structure 22, creating aphase-matching condition between the two waveguides for a wavelengthrange defined by:$\Lambda = \frac{2\pi}{\left( {\beta_{1} - \beta_{2}} \right)}$${{where}\quad \beta} = \frac{2\pi \quad n_{eff}}{\lambda}$

[0025] Λ is the period of the periodically poled structure 22, andβ_(1,2) is the propagation constant of the first or second waveguides 18and 20, which depend on the coupling wavelength λ and the averagerefractive index n_(eff) in the coupling region. Activating the gratingwill therefore enable a coupling of light of wavelength λ between thetwo waveguides. Secondly, the electric field will modify the refractiveindex in the coupling region. In the periodically poled structure, theeffective change in the alternating domains will cancel each other out,not affecting the average refractive index in the region. However, theuntouched waveguide being free of such periodic structure, the averagevalue of its refractive index will be changed by the electric field,depending on the field's strength. By selecting the amplitude of theelectric field in this waveguide, one can therefore change the value ofn_(eff), and in this manner change the wavelength for which coupling isenabled. It is a highly advantageous aspect of the present inventionthat wavelength tuning of the device is provided by simply changing thestrength of the electric field applied.

[0026] It should be noted that the present invention is not limited tothe electrode configuration illustrated above, but includes allappropriate means of generating the needed electric field. For example,as shown in the embodiment of FIG. 2, two pairs of electrodes could beprovided for each coupling region, a first pair extending on either sideof the first waveguide, and a second pair extending on either side ofthe second waveguide. This configuration advantageously allows togenerate an electric field of different values in each waveguide.Alternatively, the electrodes could be co-lateral, or the electric fieldcould be produced by a more elaborate structure. It is understood thatthe expression “electric field” used herein could be a combination ofseveral field components applied in different regions.

[0027] In the preferred embodiment, a first input 26 is connected to thefirst waveguide, assuming it is the one provided with the periodicallypoled structure 22, upstream the coupling region 24. The first input 26is for receiving, in operation, an incoming light beam A. A first output28 is similarly connected to the second waveguide 20, downstream thecoupling region 24, for exiting a light beam B resulting from thefiltering operation of the device 10. Also preferably, a second input 30and a second output 32 may respectively be connected to the secondwaveguide 20 upstream the coupling region and to the first waveguide 18downstream the coupling region, if needed by the intended use of thedevice 10. Of course, all inputs and outputs may be fiber pigtailed inorder to be useful for optical communication applications. Theextremities of the waveguides 18 and 20 connected to the second outputand input may also be left free, in which case they are preferablyangled at more than 10° to eliminate back reflections in the waveguides.

[0028] Referring to FIGS. 2, 3A, 3B and 3C, there is shown a secondembodiment of the present invention where the bandwidth of the coupledbeam is also tunable.

[0029] In this embodiment, the optoelectronic structure 12 is providedwith a third waveguide 21 in addition to first and second waveguides 18and 20. Of course, additional waveguides could be added to theoptoelectronic structure 12, if needed. A first coupling region 24 isprovided in the first and second waveguides 18 and 20, as before, and asecond coupling region 25, similar to the first one, is here provided inthe second and third waveguides 20 and 21. In both coupling regions,only one of the waveguides involved is provided with a periodic poledstructure 22. The periodic structure is preferably of a short length,preferably of less than 10 mm, which results in a relatively largebandwidth of the coupled signal, of the order of 10 nm or larger.

[0030] Means for generating a first electric field, in the firstcoupling region, are provided and preferably include pairs of electrodes36 and 38, respectively disposed on either side of the first and secondwaveguides. Similarly, a second electric field is generated in thesecond coupling region by pairs of electrodes 40 and 42. The amplitudeof both electric fields is adjustable to tune the coupling wavelength ofeach coupling region independently.

[0031] In operation, a multiwavelength optical signal is inserted intoinput 26 of the first waveguide 18. In the first coupling region 24, aportion of the input beam centered on a first coupling wavelength, andhaving a first bandwidth determined by the grating's geometry, iscoupled from the first to the second waveguides 18 and 20. The spectralprofile of the resulting beam propagating in the second waveguide 20 isschematized in FIG. 3A. When it reaches the second coupling region 25, aportion of this beam centered on a second coupling wavelength and havinga second bandwidth is coupled into the third waveguide 21, from which itexits at output 28. FIG. 3B shows the coupling spectral shape of thesecond coupling region, and FIG. 3C shows the superposition of thegraphs of FIGS. 3A and 3B, and the spectral shape of the resulting beamcoupled from the first to the third waveguides 18 and 21.

[0032] As can be seen, both the coupling wavelength and the bandwidth ofthe output beam will simply depend on the overlap between the bandwidthsof the first and second coupling regions 24 and 25. The bandwidths beingfixed values, both parameters are easily controlled by simplycalculating the required values of the first and second couplingwavelengths, and setting the amplitude of the first and second electricfields accordingly.

[0033] Referring to FIG. 4, there is shown an example of the expectedresponse of a device according to FIG. 1, when used to filter intooutput 28 a spectral portion of a beam incident at input 26. In thiscase, the interaction length between the first and second waveguides istaken to be approximately 25 mm, the distance between the waveguides isset to about 2 μm, Δβ to 6300 cm⁻¹ and Λ to approximately 10 μm. Theperiodically poled structure is made by using a voltage of 12 kV on a0.5 mm LiNbO₃ wafer for about 1 second. The expected tunability is of 30nm for an operational voltage of approximately 20 V.

[0034] One skilled in the art will readily understand that devices asdescribed above have many applications in the field of opticalcommunications. For example, in a simple embodiment it may serve as abandpass filter where only the first input 26 and first output 28 areprovided. Alternatively a second input 30 and second output 32 may beused to make a bi-directional add/drop filter, or a directional couplerwhere a signal of a given wavelength may be routed to either output 28or 32 by choosing the proper voltage. In the two latter cases, it may beadvantageous to choose a geometry where the waveguides are apart at bothends and are curved so as to come together over the coupling regiononly. In another potential application, a device according to thepresent invention may be used in an optical attenuator where the opticalpower output of a signal may be changed by tuning in or out a certainwavelength range therefrom. Other possible applications include awavelength selective optical switch, an optical modulator, etc.

[0035] Of course numerous changes could be made to the embodimentsdescribed above without departing from the scope of the invention asdefined in the appended claims.

What is claimed is:
 1. A wavelength tunable optoelectronic device,comprising: an optoelectronic structure made of a ferroelectricmaterial; a first and a second channel waveguide provided in theoptoelectronic structure, the first and second waveguides being adjacentat least along a coupling region, a periodically poled structure beingprovided in said coupling region, at least one of the first and secondchannel waveguides being untouched by said periodically poled structure;and means for generating an electric field of a selectable amplitude inthe coupling region through both the first and second waveguides, theelectric field enabling a coupling of light of a coupling wavelengthbetween the first and second waveguides in said coupling region, theamplitude of said electric field in the untouched waveguide determiningthe coupling wavelength.
 2. An optoelectronic device according to claim1, wherein the means for generating an electric field comprises a pairof electrodes.
 3. An optoelectronic device according to claim 2, whereinsaid electrodes respectively extend over and under the optoelectronicstructure, the coupling region extending therebetween.
 4. Anoptoelectronic device according to claim 1, wherein the means forgenerating an electric field comprise: a first pair of electrodesrespectively extending over and under the optoelectronic structure, aportion of the first waveguide, in the coupling region, extendingtherebetween; and a second pair of electrodes respectively extendingover and under the optoelectronic structure, a portion of the secondwaveguide, in the coupling region, extending therebetween.
 5. Anoptoelectronic device according to claim 1, further comprising: a firstinput connected to the first waveguide upstream the coupling region; anda first output connected to the second waveguide downstream the couplingregion.
 6. An optoelectronic device according to claim 5, furthercomprising a second output connected to the first waveguide downstreamthe coupling region.
 7. An optoelectronic device according to claim 6,further comprising a second input connected to the second waveguideupstream the coupling region.
 8. An optoelectronic device according toclaim 1, wherein said ferroelectric material is a LiNbO₃ crystal.
 9. Anoptoelectronic device according to claim 1, wherein said first andsecond waveguides are singlemode waveguides.
 10. A wavelength andbandwidth tunable optoelectronic device, comprising: an optoelectronicstructure made of a ferroelectric material; a first, a second and athird channel waveguide provided in the optoelectronic structure, thefirst and second waveguides being adjacent at least along a firstcoupling region, a first periodically poled structure being provided insaid first coupling region, at least one of the first and second channelwaveguides being untouched by said periodically poled structure, thesecond and third waveguides being adjacent at least along a secondcoupling region, a second periodically poled structure being provided insaid second coupling region, at least one of the second and thirdchannel waveguides being untouched by said periodically poled structure;means for generating a first electric field of a selectable amplitude inthe first coupling region through both the first and second waveguides,the first electric field enabling a coupling of light of a firstcoupling wavelength and first bandwidth between the first and secondwaveguides in said first coupling region, the amplitude of said firstelectric field in the waveguide untouched by the first periodicallypoled structure determining the first coupling wavelength; and means forgenerating a second electric field of a selectable amplitude in thesecond coupling region through both the second and third waveguides, thesecond electric field enabling a coupling of light of a second couplingwavelength and second bandwidth between the second and third waveguidesin said second coupling region, the amplitude of said second electricfield in the waveguide untouched by the second periodically poledstructure determining the second coupling wavelength; the device therebyenabling a coupling of light of a tunable wavelength and tunablebandwidth from the first to the third waveguide.
 11. An optoelectronicdevice according to claim 10, wherein each of the means for generatingthe first and the second electric fields comprises at least one pair ofelectrodes.
 12. An optoelectronic device according to claim 10, wherein:the means for generating a first electric field comprise a first pair ofelectrodes respectively extending over and under the optoelectronicstructure, the first coupling region extending therebetween; and themeans for generating a second electric field comprise a second pair ofelectrodes respectively extending over and under the optoelectronicstructure, the second coupling region extending therebetween.
 13. Anoptoelectronic device according to claim 10, further comprising: a firstinput connected to the first waveguide upstream the first couplingregion; and a first output connected to the third waveguide downstreamthe second coupling region.
 14. An optoelectronic device according toclaim 10, wherein said ferroelectric material is a LiNbO₃ crystal. 15.An optoelectronic device according to claim 10, wherein said first,second and third waveguides are singlemode waveguides.
 16. Use of anoptoelectronic device according to claim 1 as a bandpass filter.
 17. Useof an optoelectronic device according to claim 1 as an add/drop filter.18. Use of an optoelectronic device according to claim 1 as adirectional coupler.
 19. Use of an optoelectronic device according toclaim 1 as an optical switch.
 20. Use of an optoelectronic deviceaccording to claim 1 as an optical modulator.